Process for preparing multilayer plastic composites of incompatible plastics

ABSTRACT

Multilayer plastic composites containing at least two incompatible plastics, A and B, in which the layering sequence alternates between A and B, the layers of plastic B are discontinued at regular intervals, and the resulting gaps in the layers are filled in with plastic A, exhibit good adhesion between the layers. In preferred embodiments of the invention, the amorphous, crystalline, or semi-crystalline plastic B has a higher coefficient of thermal expansion than the amorphous plastic A. The plastic composite is preferably produced via coextrusion.

This is a Continuation, of application Ser. No. 08/286,514 filed on Aug.5, 1994 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to plastic composites made from at leasttwo incompatible plastics, A and B, in a multi-layer composite, in whichthe layering sequence alternates between A and B, the layers of plasticB are discontinued at regular intervals, and the resulting gaps in the Blayers are filled in with plastic A. The present invention also relatesto methods and apparatus for preparing such multilayer plasticcomposites.

2. Discussion of the Background

Plastic composites are well known. The primary goal in their productionis good adhesion between the composite elements. One of the technicallyimportant fields of this type of composites is the reinforcement ofplastic sheets using fibers or bands.

For example, DE-OS 38 35 575 describes a method for producingcontinuous, molded elements from units made of reinforcement fibers thathave been preimpregnated with thermoplastic plastics, in which thefibers are shaped by melting the thermoplastics. The preimpregnationresults in a good adhesion of the reinforcement fibers in thethermoplastic matrix.

DE-OS 38 40 374 discloses thermoplastic fiber-reinforced composites,which can be obtained by preimpregnating reinforcement fibers with amolten mass of thermoplastics, polyamides, a low-molecular weight acidamide and, if necessary, a coupling agent, along with methods forproducing these and possible applications. The composites are preparedby a process in which a mixture of thermoplastic, polyamide, acid amide,and, if necessary, a coupling agent in the form of a prefabricated filmor a freshly extruded molten film is used and, together with thereinforcement fibers, which in this case are in the form of a mat orunidirectional filament strands, are fed into a continuous mold. In thiscase, as well, good adhesion between the fibers and the thermoplasticmatrix is of great importance.

U.S. Pat. No. 4,058,581 describes a process for the continuousproduction of molded elements that are reinforced with graphite fibers,in which the individual graphite fibers are first drawn through asolution of a thermoplastic resin, preferably a polypropylene that isgrafted with acrylic acid, to effect preimpregnation with a couplingagent.

In EP-A 0,282,199, a process for the production of fiber composites madeof a thermoplastic matrix and unidirectional reinforcement fibers isdescribed, in which the fibers are preimpregnated with moltenthermoplastic. In this case, continuously parallel fiber bundles thatare not connected mechanically and a thermoplastic plastic are fed intoa twin mold, where they are subjected to great pressure at a hightemperature for a specific amount of time, after which they are cooled.In this process the fibers are completely and simultaneouslypreimpregnated, which allows a high fiber content of greater than 50% byvolume in the thermoplastic matrix. In this case as well, good adhesionbetween the fibers and the thermoplastic matrix is of primaryimportance.

EP-PS 0,407,852 describes sheets made of acrylic glass that are suitablefor use as sound-proofing elements, and which have embedded in them,approximately in the center, monofilament synthetic fibers, or a latticeweb made of such fibers, which are preferably polyamide fibers. Thepolyamide fibers show no appreciable loss in tear resistance, since theadhesion between these types of fibers and the acrylic glass thatsurrounds them is relatively light.

Up to EP-PS 0,407,852, the patent specifications and publishedapplications referred to above describe exclusively plastic compositesthat include layers of coupling agents. The use of additional couplingagents generally necessitates great technical expenditure, for exampleon feed devices for the coupling agents or a special melting and/orextrusion guiding device in the case of coextruded plastic composites.In addition, the adhesion that can be obtained in the plastic compositeover layers of coupling agent is limited.

Thus, there remains a need for a method for combining incompatibleplastics, and more importantly their mechanical properties, without theuse of coupling agents. The application referred to above, EP-PS0,407,852, attains this object only to a limited extent and in a verycomplex manner, and the discontinuous production of the plasticcomposites made of acrylic glass and synthetic fibers is very costly.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide novelmultilayer plastic composites.

It is another object of the present invention to provide multilayerplastic composites which contain layers of at least two incompatibleplastics.

It is another object of the present invention to provide multilayerplastic composites which contain layers of at least two incompatibleplastics and which exhibit improved mechanical properties.

It is another object of the present invention to provide a novel methodfor preparing such multilayer plastic composites.

It is another object of the present invention to provide novel apparatusfor preparing such multilayer plastic composites.

These and other objects, which will become apparent during the followingdetailed description, have been achieved by the inventors' discoverythat multilayer plastic composites made of at least two incompatibleplastics, A and B, can be produced if the layering sequence alternatesbetween A and B, and if the layer of plastic B is discontinued atregular intervals and the resulting gaps in the layer are filled in withplastic A. In a preferred embodiment of the invention, the layers ofplastic B are inserted as unidirectional columns in the plastic A.

In a further preferred embodiment of the invention the plastic B has agreater thermal expansion than the plastic A, whereby the plastic A ismost preferably amorphous and the plastic B is most preferablyamorphous, crystalline, or semi-crystalline. The composites arepreferably produced via coextrusion, in which the plastic B is mostpreferably processed in an oscillating stream, which results in periodicchanges in the cross-sections of the layer segments made of plastic B.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same become betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates a first embodiment of the present multilayer plasticcomposite;

FIG. 2 illustrates a second embodiment of the present multilayer plasticcomposite; and

FIG. 3 illustrates an embodiment of the apparatus useful for producingthe present multilayer plastic composite.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The thermoplastic plastics A and B are chosen such that the blending ofthe two polymers (plastics) is fundamentally incompatible. Incompatiblepolymer blends are designated, for example, as mechanical polymer blends(see, e.g., Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. Ed.,Vol. 18, pages 443 to 447, Wiley Interscience, NY, 1982). This placesvery few limitations on the possible selections for plastics A and B.

Preferably, the thermoplastic plastic A is amorphous and has a lowercoefficient of thermal expansion than the plastic B, which can beamorphous, semi-crystalline, or crystalline. The plastic B preferablyhas greater elongation at a break (fracture), greater transversalstability, and a higher viscosity than the matrix plastic A.

The following are possible examples of amorphous plastics for A or B:polyvinyl ester, polyvinyl ether, amorphous polyvinyl halogenides,polystyrenes, polyphenylene oxides, polyphenylene sulphides,polycarbonates, polysulphones, amorphous polyamides, polyether ketones,polyether ether ketones, polyether sulphones, polyimides,polyetherimides, polyfluoroalkenes, polyester carbonates, amorphouspolyolefins, and most particularly preferred, poly(meth)acrylates.

Examples of crystalline or semi-crystalline B plastics are polymerswhose crystallinity is contingent upon a uniform tacticity or enoughsmall substituents, to permit at least the partial formation of acrystal lattice. Examples of such polymers include polyester,crystalline polyolefins, crystalline polyvinyl halogenides, liquidcrystalline polymers having mesogenic groups in the main and/or lateralchains, or most preferably, crystalline polyamides. Regarding theproduction and characterization of the A and B plastics, see, forexample, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. Ed.,Vol. 18, pages 720 to 755, Wiley Interscience, NY, 1982.

There is no particular limitation on the molecular weight of thethermoplastics A and B, so long as the thermoplastics A and B can beprocessed to form the present multilayer composites. Typically, thethermoplastic A and B will have number average molecular weights of 10³to 10⁶ daltons, preferably 10⁴ to 5×10⁵ daltons.

FIG. 1 illustrates the fundamental structure of a three-layer plasticcomposite, comprised of an amorphous plastic A and an amorphous,crystalline, or semi-crystalline plastic B, having the layering sequenceA-B-A. The B layer is discontinued at regular intervals, and theresulting gaps are filled in with plastic A, thereby ensuring a strongadhesion between the two A layers.

In the embodiment shown in FIG. 1, the multilayer composite is in theform of a flat sheet. However, it is to be understood that the compositemay also take the form of other shapes, such as a hollow pipe, curvedsheet, etc.

In the embodiment shown in FIG. 1 two layers of plastic A are sandwichedabout a single layer of B. However, it is to be understood that thecomposite may contain additional layers, so long as the layers ofplastic A and plastic B alternate. In general, for every n layers ofplastic B there will be n+1 layers of plastic A. Although, there is noparticular limit on the number of layers of A and B in the presentcomposite, for the sake of convenience of manufacture, it is preferredthat n be less than 3, and it is particularly preferred that n be 1.

Although there are no particular limitations on the absolute or relativedimensions of the layers of A and B, in embodiments in which two layersof A are sandwiched about a single layer of B, such as shown in FIG. 1,the overall thickness of the composite is typically 0.5 to 25 mm,preferably 1 to 20 mm; the thickness of the B layer is 0.1 to 22 mm,preferably 0.2 to 20 mm; the gaps in the B layer are 1 to 100 mm,preferably 5 to 50 mm, wide; and the distance between nearest neighborgaps in the B layer (cross-sectional width of individual segments in Blayer) is 1 to 50 mm, preferably 2 to 30 mm. Of course, in embodimentscomprising additional layers of A and B, the overall thickness of themultilayer composite will increase in proportion to the number ofadditional layers of A and B.

In a preferred embodiment, the plastic B has a greater coefficient ofthermal expansion than the plastic A. As a result, when the layers ofthe plastic B, which are embedded in the plastic A, cool, the greaterthermal shrinkage of the plastic B causes the layers of the plastic B tobecome tightened. This results in advantageous mechanical properties inthe plastic composite, such as a greater resistance to fracture, greatertransverse strength, greater dimensional stability, and the preventionof shattering under impact stress.

A further advantageous embodiment of the invention involves an insertionof segmented layers of the plastic B, the cross-sections of which varyperiodically, resulting in an excellent anchoring of the layers of theplastic B in the plastic A (see FIG. 2). In this case, as before, thetightening of the layers of the plastic B upon cooling is increasedsubstantially, which adds to the mechanical advantages of the plasticcomposite already outlined above.

In the embodiment shown in FIG. 2, there is no particular limit on thedegree of variation of the cross-sectional width of the segments inlayer B. However, good results have been achieved when the variation is5 to 100%, preferably 5 to 50%, based on the minimal cross sectionalwidth of an individual segment of layer B, and the period of thisvariation is typically 10 to 100 mm, preferably 20 to 50 mm.

The present multilayer composites may be used, e.g., as window panes. Ifa transparent plastic A and a non- or semi-transparent plastic B areused, the result is a plastic composite having a shading effect, whichcan be used in the manufacturing of sun shades.

By using varying colors of plastics A and B, plastic composites for usein luminous advertising or for other possible applications in thelighting and decorating field can be produced.

The plastic composites of the present invention, made preferably ofamorphous plastic A and amorphous, crystalline, or semi-crystallineplastic B, are preferably produced relatively simply in a singleproduction stage via coextrusion.

FIG. 3 illustrates the main section of the device used in producing thepresent composites, the so-called coextrusion die. In the die shown inFIG. 3, the plastic A is fed in at the feed points (2) and is thendispersed in two separate distribution channels (5) to the desiredproduct width. Between the distribution channels (5) for the plastic A,the plastic B is fed at the feed point (3) into a bore hole (6) that hasa lateral distribution. The plastic B exits this bore hole via a largernumber of bore holes (7), which correspond to the number of segments ofB in the A matrix. At the point at which B emerges from the bore holes,the two layers of A are also joined, thereby enclosing the columns ofplastic B.

The cross-sectional shape of the B segments in A is largely dependentupon the shape of the bore holes (7) at their points of efflux and therelative viscosities of the plastics A and B. If the viscosity of B isgreater than the viscosity of A, then the segments of B and A will bemore round cross-sectionally; if these viscosity ratios are reversed,the segments will be more flat.

After it has exited the die, the composite can be sized using aconventional smoothing device. Because the plastic B is enclosed in theplastic A, it cools relatively slowly. If the plastic B issemi-crystalline or crystalline, this process results in substantiallygreater degrees of crystallinity than if the B columns had been producedvia conventional extrusion. If a periodic variation in thecross-sectional width of the B segments over their entire length isdesired, this can be easily achieved by periodically altering the feedrate of the stream of the plastic B that is fed into the feed points(3). Because the plastic B has a higher thermal expansion than theplastic A, the B segments become tempered with the mold closure betweenthe two plastics (comparable with reinforced concrete). This permits afurther improvement in the mechanical properties of the composite.

Other features of the invention will become apparent in the course ofthe following descriptions of exemplary embodiments which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLES

If a steel ball bearing having a diameter of 100 mm (weight approx. 4kg.) is allowed to impact on an 8 mm thick, flat sheet made ofpolymethyl methacrylate (PMMA, Plexiglass® 7H, Man. Röhm) from a heightof 1.5 m at a 45 degree angle, the ball bearing will break through thesheet, forming many large splinters.

If the same experiment is conducted using a sheet as specified in theinvention, in which the plastic A is PMMA (Plexiglass® 7H Man. Röhm,coefficient of thermal expansion α₁=1.8×10⁻⁴ K⁻¹ at 120° C.) and theplastic B is polyamide 12 (Vestamide® 1852 Man. Hüls AG, coefficient ofthermal expansion α₂=2.7×10⁻⁴ K⁻¹ at 120° C.), and in which the Bsegments are spaced at 30 mm intervals and have an oval cross-sectionwith an average diameter of 3 mm, then the sheet will only be crackedsuperficially and dented toward the rear at the point of impact of theball bearing; the ball bearing will be halted and no splinters willform.

If the height of fall of the ball bearing is increased to 2.2 m for thesheet specified in the invention, the ball bearing will break throughthe sheet. However, all splinters will be contained. The propertiesdescribed can be further improved by using B segments having across-section that varies periodically over the length of the segmentsand a resulting mold closure between A and B.

The coefficient of thermal expansion a is determined in accordance withDIN 53752.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for producing a multilayer plasticcomposite, said composite comprising a sequence of layers of at leasttwo incompatible thermoplastic plastics, A and B, wherein said sequenceof layers alternates between A and B, a layer of plastic B isdiscontinuous at regular intervals to form gaps in said layer of plasticB, and said gaps in said layer of B are filled in with plastic A; saidmethod comprising coextruding plastic A and plastic B through a diecomprising a pair of parallel exit slits and a plurality of exit ports,with a gap between each port, located between said pair of parallel exitslits, said plurality of exit ports being evenly spaced along a lineparallel to said pair of exit slits, wherein said coextruding plastic Aand plastic B comprises forming a pair of fluid streams of said plasticA by passing a fluid stream of plastic A through said pair of parallelexit slits and forming a plurality of fluid streams of plastic B withgaps between each stream of said plastic B by passing a fluid stream ofplastic B through said plurality of exit ports, so that said fluidstreams of said plastic A exit said pair of exit slits and said fluidstreams of plastic B exit said plurality of exit ports in such a mannerto result in a portion of said fluid streams of said plastic A passingthrough said gaps between each stream of said plastic B to effect fusionof said pair of fluid streams of said plastic A, to obtain saidcomposite.
 2. The method of claim 1, wherein said plastic B has agreater coefficient of thermal expansion than said plastic A.
 3. Themethod of claim 1, wherein said plastic A is amorphous, and said plasticB is semi-crystalline.
 4. The method of claim 1, wherein said plastic Ais amorphous, and said plastic B is crystalline.
 5. The method of claim1, wherein said plastic A is amorphous, and said plastic B is amorphous.